PHP 8+ Example for hook_user_presave causes Deprecation notice if data field is NULL or FALSE

[yorkshire-pudding]

Description of the bug

Following the example of hook_user_presave() will cause deprecation notices with standard error reporting and "502 Bad Gateway" if Devel is installed and Krumo backtrace is enabled.

Here is the doc and example code from user.api.php:

/**
 * Act on a user account being inserted or updated.
 *
 * This hook is invoked before the user account is saved to the database.
 *
 * Modules that want to store properties in the serialized {users}.data column,
 * which is automatically loaded whenever a user account object is loaded, may
 * add their properties to $account->data in order to have their data serialized
 * on save.
 *
 * @param $account
 *   The user account object.
 *
 * @see hook_user_insert()
 * @see hook_user_update()
 */
function hook_user_presave($account) {
  // Make sure that our form value 'my_module_foo' is stored as
  // 'my_module_bar' in the 'data' (serialized) column.
  if (isset($account->my_module_foo)) {
    $account->data['my_module_bar'] = $account->my_module_foo;
  }
}

It is feasible to not have any data in the data field for a user:

If a module invokes hook_user_presave() in the way suggested on one of these users then if deprecation notices are enabled they will be shown or if Devel Krumo backtrace is enabled for deprecations then a Bad Gateway error can be caused.

Steps To Reproduce

To reproduce the behavior:

1. Enable all error messages
2. Enable deprecations to be logged
3. Install and enable Devel and Devel Generate and enable the Krumo Backtrace above rendered page
4. Use Devel Generate to create some users
5. Install and enable Matomo Analytics (1.x-2.12.3) (I'll be raising an issue there too but keep in mind this is following the current example)
6. Edit one of those users and edit something like the email address
7. Save

Actual behavior

If Devel Krumo backtrace is enabled then Bad gateway error (on on the b.org demo a tubgoat)
If not then a deprecation notice:

Deprecated function: Automatic conversion of false to array is deprecated in matomo_user_presave() (line 456 of /var/lib/tugboat/backdrop/modules/matomo/matomo.module).

Expected behavior

User saves with no deprecation notice.

Additional information

Add any other information that could help, such as:

• Backdrop CMS version: 1.31.0
• Web server and its version: Apache
• PHP version: 8.2
• Database sever (MySQL or MariaDB?) and its version: n/a
• Operating System and its version: n/a
• Browser(s) and their versions: n/a

EDIT: Moved generate users to before installing Matomo

[argiepiano]

This LGTM. @indigoxela had suggested using a !is_array($account->data) check instead of (!$account->data). That would avoid unnecessarily making the property into an array when it already was an array. However I do see a check similar to this PR already in core in User::preSave() meaning that this approach is also fine. Either way works.